On Tue, Apr 17, 2018 at 12:57:12PM +0100, James Bottomley wrote: > On Tue, 2018-04-17 at 04:47 -0700, Matthew Wilcox wrote: > > On Tue, Apr 17, 2018 at 10:13:34AM +0100, James Bottomley wrote: > > > On Sat, 2018-04-14 at 17:41 -0700, Matthew Wilcox wrote: > > > > On Sat, Apr 14, 2018 at 06:44:19PM -0400, Theodore Y. Ts'o wrote: > > > > > What needs to happen is freelist should get randomized much > > > > > later in the boot sequence. Doing it later will require > > > > > locking; I don't know enough about the slab/slub code to know > > > > > whether the slab_mutex would be sufficient, or some other lock > > > > > might need to be added. > > > > > > > > Could we have the bootloader pass in some initial randomness? > > > > > > Where would the bootloader get it from (securely) that the kernel > > > can't? > > > > In this particular case, qemu is booting the kernel, so it can apply > > to /dev/random for some entropy. > > Well, yes, but wouldn't qemu virtualize /dev/random anyway so the guest > kernel can get it from the HWRNG provided by qemu? The part of Ted's mail that I snipped explained that virtio-rng relies on being able to kmalloc memory, so by definition it can't provide entropy before kmalloc is initialised. > > I thought our model was that if somebody had compromised the > > bootloader, all bets were off. > > You don't have to compromise the bootloader to influence this, you > merely have to trick it into providing the random number you wanted. > The bigger you make the attack surface (the more inputs) the more > likelihood of finding a trick that works. > > > And also that we were free to mix in as many untrustworthy bytes of > > alleged entropy into the random pool as we liked. > > No, entropy mixing ensures that all you do with bad entropy is degrade > the quality, but if the quality degrades to zero (as it might at boot > when you've no other entropy sources so you feed in 100% bad entropy), > then the random sequences become predictable. I don't understand that. If I estimate that I have 'k' bytes of entropy in my pool, and then I mix in 'n' entirely predictable bytes, I should still have k bytes of entropy in the pool. If I withdraw k bytes from the pool, then yes the future output from the pool may be entirely predictable, but I have to know what those k bytes were.
